Thermal tempering and chemical treatments by ion exchange are widely recognized means for strengthening glass. Glasses strengthened by such processes have compressive stress in the surface layers and tensile stress in the bulk.
Glasses that are either tempered or chemically strengthened are difficult, if not impossible, to cut or separate into pieces of desired shape and/or sizes. Cutting operations are therefore performed before carrying out strengthening operations. Conventional score-and-break techniques do not work because the initial crack does not propagate along the score line, but instead tends to bifurcate multiple times. Consequently, the glass sample usually breaks into multiple pieces.